For most people, your card settles in the currency of the country where you opened it. Opened in China → CNY; US → USD; Eurozone → EUR. If unsure, check with the methods below.
3 ways to check
Check your statement: the currency your monthly bill is issued/repaid in is your card's currency.
Cardholder agreement / fee schedule: search for "billing currency" and "currency conversion fee".
Ask support: "What currency do foreign purchases post in, and is there a conversion fee?"
Note: a few are multi-currency cards (may settle in USD) or foreign-currency accounts — use the actual settlement currency. Once you know it, enter it as the "card currency" in the calculator.